Microporous membranes are generally defined as thin walled structures having an open spongy morphology with a narrow pore size distribution. The mean pore size for microporous membranes typically range from 0.01 .mu.m to 10 .mu.m. Traditionally, microporous membranes are used to remove fine particulate matter such as dust and bacteria from liquids and gases. The filtration mechanism is believed to be a combination of size exclusion and absorption/adsorption on the walls of the pores inside the membrane. In general, the smaller the pore size of membrane, the slower the rate of membrane at which a filtrate can be passed. To be considered "microporous", the typical inner width of the membrane pores is in the range that passes macromolecules and retains particles contained in a fluid. Below this range, are "ultrafiltration" (UF) membranes which serve to retain macromolecules such as albumin, and "reverse osmosis" (RO) membranes which serve to separate ions. To be useful for a particular application, the fluid flow rate through the membrane must be reasonably high.
In aqueous microfiltration such as the production of particle free water or solutions in electronics, beverage, and pharmaceutical industries, it is highly desirable to use filtration membranes which have as little leachable material as possible. Also it is desired that the membranes be easily wettable by water or aqueous solutions. Therefore, membranes which are inherently water wettable are often preferred over hydrophobic membranes post-treated by a wetting agent. Thus, hydrophobic membranes are commonly made wettable by adding a wetting agent. Upon use of the treated hydrophobic membranes, however, there is a risk that wetting agent may be leached by the solution being filtered. Such leaching, in turn, may result in contamination of the filtrate.
Polyethersulfone (PES) resin has been used to prepare microporous membranes as described in U.S. Pat. No. 4,900,449. The membranes contain polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) as non-leachable, intrinsic wetting agents and therefore are hydrophilic. Recently, a new microporous membrane comprising PES resin and phenoxy resin has been developed as described in copending U.S. patent application Ser. No. 531,237. This membrane has a substantially higher strength than the membrane made with only PES resin, and can be readily used in disc form as a housed porous filter membrane component, in a melt-compatible thermoplastic device for the membrane such as a device of the type disclosed in U.S. Pat. No. 4,444,661. Despite the unexpectedly high strength, this membrane is however, hydrophobic.
Polyethersulfone has been shown to be miscible in a common solvent (dimethylformamide or dimethylsulfoxide), with phenoxy resin (V.B. Singh and D.J. Walsh, J. Macromol, Sci.-Phys., B25 (1 & 2), 65-87, 1986). Also shown is that the melting temperature of cast films of blended PES/phenoxy resin is lowered by using more phenoxy resin (relative to PES) in the blend. Not suggested is a membrane made from such a blend nor was such a film suggested to be porous or to be useful as a filtration membrane.
Poly-2-oxazoline, more particularly poly(2-ethyl-2-oxazoline), is described as being a water soluble adhesive by Chiu et al., Advances in Chemistry Series 213, pp. 425-433, American Chemical Society, 1986. Poly-2-oxazoline is used to blend with various thermoplastics to prepare membranes for use in separating components of liquid mixtures such as water/ethanol or ethanol/hexane mixtures as described in U.S. Pat. No. 4,678,833. In this patent, it is reported that the membranes made from poly-2-oxazoline are sometimes extractable from miscible blends with alcohol or alcohol/water mixtures. Therefore, such leaching may not only result in contamination of the filtrate, but also cause the membrane to be non-wettable when re-used in aqueous solutions.